It has become apparent that under certain running conditions, as soon as the implement being drawn begins to apply a not insignificant force resisting forward travel, for example under the effect of an element, such as a disc, of the said implement digging into the ground, the tractor begins to bounce and hop initially causing the driver problems concerned with comfort, and therefore making the driver's work even more difficult, but which could go so far as to cause the tractor problems with making forward progress as the contact periods become too short to provide the friction necessary to allow the tractor to advance over the land, thus causing the said tractor to lose efficiency by degrading the grip of the tyre on the ground concerned.
This bouncing and jolting is characterized by intermittent or almost-sustained oscillations of the rolling assembly at low frequencies usually ranging between 0.5 and 4 Hz, and is known by the name of “power hop”.
The solutions that users currently employ involve making the tractor heavier and/or altering the tractor tyre inflation pressure empirically or using interactive methods suggested by the vehicle constructors or the tyre manufacturers. This adjusting of the pressure or of the tractor ballast weight causes the user to lose time and also detracts from the performance of the vehicle because the recommended optimum settings are no longer observed. Excessively high pressures may, for example, lead to greater compaction of loose soil and to even greater resistance to forward travel.
Extensive research conducted by the applicant company into modifications to tyres and, in particular, the meridian profiles of the tread, the meridian profiles of the carcass reinforcement, the carcass and crown reinforcement materials, modifications to the patterns and dimensions of the tread block have not, as yet, yielded the hoped-for improvements because these modifications have little or no impact on the phenomenon as described herein above.
As far as the conventional design of agricultural machinery tyres is concerned, the carcass reinforcement, anchored into each bead, is made up of at least one layer of textile and/or metal reinforcing elements, the said elements being substantially parallel to one another in the layer and potentially being substantially radial and/or distinctly crossed from one ply to the next making equal or unequal angles with the circumferential direction. The carcass reinforcement is usually surmounted by a crown reinforcement made up of at least two working crown layers of reinforcing elements which may be made of textile or of metal, but which are crossed from one layer to the next, making small angles with the circumferential direction. The tread of the tyre in question is formed of rubber blocks or strips which are inclined with respect to the circumferential direction by an angle which is generally high, and which are generally circumferentially separated from one another by hollows of a width (measured in the circumferential direction) that exceeds the average width of the strips. The said strips may be symmetrical with respect to one another about the equatorial plane, being axially continuous or, as in the majority of cases, axially discontinuous. The ends of the strips that are axially close to the equatorial plane are then in most cases circumferentially offset from one another, exhibiting what is commonly known as a chevron pattern.
In patent FR 1 046 427, the applicant company has, for example, described carcass reinforcements in which the directions of the reinforcing elements of the ply or plies in a sidewall are substantially symmetrical about the equatorial plane of the tyre, with the directions of the reinforcing elements of the said ply (plies) in the other sidewall. As disclosed and explained, a ply is to be understood to mean either a ply that is axially continuous from bead to bead, or two half-plies anchored in each bead to an annular bead reinforcing element but the radially upper ends of which are distant from one another and from the equatorial plane.
U.S. Pat. No. 3,108,628 again teaches, with a view to giving the tyre good stability under transverse load, supplementing the radial carcass reinforcement with half-plies known as stabilizing half-plies and which are formed of reinforcing elements that are inclined with respect to the circumferential direction, the said half-plies overlapping at the crown over most of the axial width of the tread so that the reinforcing elements cross one another.
Patent FR 1 259 199 also discloses and describes a directional carcass reinforcement made up of two half-plies of reinforcing elements which are inclined with respect to the circumferential direction so that they overlap at the crown of the tyre to form a crossed reinforcement.
Other documents describe solutions which are not tyre-related but are related to the vehicle and/or to the drawn implement in an attempt to alleviate this bouncing problem.
Document U.S. Pat. No. 6,260,873 describes a device interposed between a tractor and a drawn implement to damp out the tensions that may arise between the tractor and the implement.
Document US 20050269796 describes an optimized suspension system for an agricultural tractor limiting this power hop phenomenon.
The various solutions already proposed do not guarantee full effectiveness on all types of soil or, alternatively, are able only to attenuate, with varying degrees of permanence, the damping of these bouncing phenomena, at the expense of other desired vehicle performance properties.